The present invention relates to optical communication generally, and more particularly to optical communication utilizing wavelength division multiplexing (WDM).
Today, the telecommunication industry is experiencing growth in demand for communication services and such demand is further expected to grow in the future. One of the ways to meet such demand is by expanding capacity of information carried over fiber optic cables in optical communication systems. One of the most advanced ways of achieving such expansion today is by using wavelength division multiplexing (WDM) for transmitting simultaneously multiple signals at different wavelengths over the same fiber optic cable.
The concept of WDM is based on the theory that a discrete light frequency can carry its own unique package of information. Thus, for example, two separate frequencies that carry data can be combined and transmitted in a combined form along a fiber optic cable to a receiving end. At the receiving end the two frequencies can be received and separated, and the data carried by each separate frequency can be regenerated. Based on this concept, the first systems that employed WDM multiplexed and demultiplexed signals at wavelengths of 1310 nanometer (nm) and 1550 nm (1.0 nanometer is 1.0*10**xe2x88x929 meter or 1.0**xe2x88x923 micrometer).
The advent of wide-band optical amplifiers made WDM of many wavelengths onto the same fiber optic cable practical since a plurality of wavelengths in an operating bandwidth of a wide-band optical amplifier could be amplified simultaneously by the same amplifier. With wide-band optical amplifiers being available, communication over long distances can be implemented.
A common wide-band optical amplifier which is commercially available today is the Erbium Doped Fiber Amplifier (EDFA) which has an operating bandwidth around 1550 nm. Wide-band optical amplifiers at 1310 nm are not yet commercially available today, although development of such optical amplifiers continues.
Other elements of a WDM based system, such as multiplexers/demultiplexers, optical transmitters, optical receivers and tunable optical filters, exist today and are commercially available. However, there are still several problems that have to be resolved in order to provide reliable optical communication systems which utilize WDM.
One of the problems which typically arises in systems utilizing dense wavelength division multiplexing (DWDM) of several tens of channels or more relates to spacing of channel wavelengths. If channel wavelengths are not spaced an adequate distance apart, drifts in wavelength characteristics of optoelectric components over time may cause interference between channel wavelengths. The drifts may be generated, for example, due to a change in temperature. Such drifts are typically of a low frequency type.
Other problems relate to wavelength stability of the optical transmitters, and to degradation and interference effects arising from use of non-ideal fiber optic cables. Such effects include, for example, dispersion, self-phase modulation, and cross-phase modulation.
Descriptions of optical communication systems utilizing WDM and of elements of such communication systems are found in the following publications:
An article titled xe2x80x9cMining the Optical Bandwidth for a Terabit per Secondxe2x80x9d, by Alan Eli Willner, in IEEE Spectrum, April 1997, pp. 32-41;
An article titled xe2x80x9cRecord Data Transmission Rate Reported at ECOC 96xe2x80x9d, by Paul Mortensen, Laser Focus World, November 1996, pp. 40-42;
An article titled xe2x80x9cMultiple Wavelengths Exploit Fiber Capacityxe2x80x9d, by Eric J. Lemer, Laser Focus World, July 1997, pp. 119-125;
An article titled xe2x80x9cAdvances in Dense WDM Push Diode-Laser Designxe2x80x9d, by Diana Zankowsky, Laser Focus World, August 1997, pp. 167-172;
An article titled xe2x80x9cMultistage Amplifier Provides Gain Across 80 nmxe2x80x9d, by Kristin Lewotesky, Laser Focus World, September 1997, pp. 22-24;
The Communications Handbook, CRC Press and EEE Press, 1997, Editor-in-Chief Jerry D. Gibson, Section 65, pp. 883-890; and
An article titled xe2x80x9cWDM Local Area Networksxe2x80x9d, by Kazovsky et al., IEEE LTS, May 1992, pp. 8-15.
Additionally, in U.S. Pat. No. 5,170,273 to Nishio there is described a cross-talk reducing optical switching system which receives electrical digital signals at its input terminal.
U.S. Pat. No. 5,191,457 to Yamazaki describes a VWDM optical communication network in which optical beams are modulated by channel discrimination signals of different frequencies.
U.S. Pat. No. 5,194,977 to Nishio describes a wavelength division switching system with reduced optical components using optical switches.
U.S. Pat. No. 5,557,439 to Alexander et al. describes wavelength division multiplexed optical communication systems configured for expansion with additional optical signal channels.
U.S. Pat. No. 5,680,490 to Cohen et al. describes a comb splitting system which demultiplexes and/or multiplexes a plurality of optical signal channels at various wavelengths.
U.S. Pat. No. 5,712,932 to Alexander et al. describes reconfigurable wavelength division multiplexed systems which include configurable optical routing systems.
The disclosures of all references mentioned above and throughout the present specification are hereby incorporated herein by reference.
The present invention seeks to provide a method and system for improving communication performance in optical communication systems utilizing WDM.
In the present invention, an optical communication system utilizing WDM converts data provided over up to N channels to optical signals which are communicated over at least some of N channel wavelengths corresponding to the N channels. The N channel wavelengths are spaced S1 nm from each other.
Communication performance of the optical communication system may be improved in any of the following cases and in any combination thereof:
(1) not all the N channels are carrying data simultaneously and communication performance of some channels that carry data is low;
(2) time dependent changes, such as temperature changes, cause degradation in communication performance; and
(3) actual capacities of at least some of the N channels are lower than a maximum attainable channel capacity and communication performance of some of the channels that carry data is low.
In the case that the optical communication system is only partially loaded, i.e. only K out of the N channels carry data, the K channels carrying data are detected at a switching unit which forms part of the optical communication system.
After the K channels carrying data are detected at the switching unit, a controller at the switching unit calculates a channel spacing S2 which is greater than S1. Additionally, the controller also computes a number NCW, where NCW characterizes a distribution of channel wavelengths in which NCW channel wavelengths in a sub-group of K channel wavelengths corresponding to the K channels are spaced at least S2 nm from at least one nearest neighbor channel wavelength in the sub-group of K channel wavelengths.
After computing the values of NCW and S2, tile controller uses the values of NCW and S2 to select a sub-group of K optical transmitters to be used for transmission of the data carried over the K channels. Selection of the sub-group of K optical transmitters is performed by sequentially determining a number of NCW optical transmitters which transmit at channel wavelengths spaced S2 nm from each other, and then determining the rest of K-NCW optical transmitters which transmit at channel wavelengths that are not spaced S2 nm from each other.
After determination of the sub-group of K optical transmitters, the controller provides to a router control signals identifying the selected sub-group of K optical transmitters. The router routes the data carried over the K channels to the sub-group of K optical transmitters for transmission thereby.
The selection of the sub-group of K optical transmitters generally enables transmission of the data over channel wavelengths spaced at an increased channel spacing with respect to an initial channel spacing. This reduces interference effects and improves communication performance.
In the case that time dependent changes, such as temperature changes, cause degradation in communication performance, K out of N channels that carry data are detected and routed to K optical transmitters for transmission of the data carried over the K channels from a transmitting end of the optical communication system to a receiving end of the optical communication system. The transmission is performed by the K optical transmitters over K channel wavelengths corresponding to the K channels.
If a quality of reception of data received over at least one of the K channel wavelengths at the receiving end is determined to be below a threshold, a reception indicator indicating that the quality of reception of data received over at least one of the K channel wavelengths at the receiving end is below the threshold is transmitted back to the transmitting end.
When the reception indicator is received at the transmitting end, a channel hop is performed from the at least one of the K channel wavelengths to at least one unoccupied channel wavelength at the transmitting end and at the receiving end in synchronization.
If, in addition to a low quality of reception of data received over at least one of the K channel wavelengths at the receiving end, actual capacities of at least some of the N channels are lower than a maximum attainable channel capacity, a bit rate of the data carried over the at least one of the K channels whose reception is determined to be below the threshold may be reduced at the transmitting end in addition to performance of the channel hop. Alternatively, channel hop may be prevented, and improvement of communication performance may be achieved by reduction of the bit rate only.
The optical communication system may also employ different wavelength bands for transmission of data signals and control signals. The data signals are generated at different wavelengths in a wavelength range between 1.52 micrometer and 1.62 micrometer. The control signals are generated at a wavelength around 1.31 micrometer. Then, the control signals and the data signals are multiplexed in a wavelength division multiplexer for transmission in a wavelength division multiplexed form.
When the optical communication system is used in full capacity, a channel hop, if required, may be performed from a data channel in the wavelength range between 1.52 micrometer and 1.62 micrometer to the control channel operating at a wavelength around 1.31 micrometer.
There is thus provided in accordance with a preferred embodiment of the present invention a method of transmitting data carried over K channels via an optical transmission system including a group of N optical transmitters transmitting at channel wavelengths spaced at least S1 nm from each other, where K is less than N, the method including detecting the K channels carrying data, and routing the K channels to a sub-group of K optical transmitters selected from the group of N optical transmitters to provide a distribution of the K optical transmitters in which a highest number of optical transmitters in the sub-group transmit at channel wavelengths spaced at least S2 nm from channel wavelengths used by at least one nearest neighbor optical transmitter in the sub-group, where S2 is greater than S1. Preferably, S2=S1*(N/K+1).
Additionally, the method also includes the steps of determining that a quality of reception of data received over at least one of K channel wavelengths at a receiving end is below a threshold, wherein the K channel wavelengths respectively correspond to the K channels, transmitting to the optical transmission system in response to the determining step, a reception indicator indicating that the quality of reception of data received over the at least one of the K channel wavelengths at the receiving end is below the threshold, and hopping from the at least one of the K channel wavelengths to at least one unoccupied channel wavelength at the transmission system and at the receiving end in synchronization.
Preferably, the at least one unoccupied channel wavelength is spaced at least S2 nm from the at least one of the K channel wavelengths.
Further preferably, each optical transmitter in the group of N optical transmitters operates at a wavelength in a wavelength range between 1.52 micrometer and 1.62 micrometer, and the transmitting step includes the step of transmitting the reception indicator over a control channel operating at a wavelength around 1.31 micrometer.
Preferably, each optical transmitter in the group of N optical w transmitters operates at a wavelength in a wavelength range between 1.52 micrometer and 1.62 micrometer, and the hopping step includes the step of hopping from the at least one of the K channel wavelengths to at least one channel wavelength centered around 1.31 micrometer at the optical transmission system and at the receiving end in synchronization.
Additionally, the method also includes the steps of determining that a quality of reception of data received over at least one of K channel wavelengths at a receiving end is below a threshold, wherein the K channel wavelengths respectively correspond to the K channels, transmitting to the optical transmission system, in response to the determining step, a reception indicator indicating that the quality of reception of data received over the at least one of the K channel wavelengths at the receiving end is below the threshold, and reducing a data rate of the data carried over the at least one of the K channel wavelengths at the optical transmission system.
The method also preferably includes the step of condensing L radio frequency (RF) channels carrying data into the K channels prior to the detecting step, wherein L is greater than K and less than or equal to N. Preferably, the condensing step includes the step of transferring data carried over Lxe2x88x92K channels to the K channels thereby increasing a data rate of data carried over at least one of the K channels.
There is also provided in accordance with a preferred embodiment of the present invention a method of transmitting data carried over K out of N channels to be employed in an optical data communication system adapted to use N optical transmitters for transmitting information from a transmitting end of the communication system to a receiving end of the communication system using WDM, the method including the steps of detecting the K channels carrying data, routing the K channels to K optical transmitters for transmission of the data carried over the K channels by the K optical transmitters over K channel wavelengths corresponding to the K channels, wherein the K optical transmitters are selected from the N optical transmitters, determining that a quality of reception of data received over at least one of the K channel wavelengths at the receiving end is below a threshold, transmitting to the transmitting end, in response to the determining step, a reception indicator indicating that the quality of reception of data received over the at least one of the K channel wavelengths at the receiving end is below the threshold, and hopping from the at least one of the K channel wavelengths to at least one unoccupied channel wavelength at the transmitting end and at the receiving end in synchronization.
Preferably, each of the N optical transmitters operates at a wavelength in a wavelength range between 1.52 micrometer and 1.62 micrometer, and the transmitting step includes the step of transmitting the reception indicator over a control channel operating at a wavelength around 1.31 micrometer.
Further preferably, each of the N optical transmitters operates at a wavelength in a wavelength range between 1.52 micrometer and 1.62 micrometer, and the hopping step includes the step of hopping from the at least one of the K channel wavelengths to at least one channel wavelength centered around 1.31 micrometer at the transmitting end and at the receiving end in synchronization.
Additionally, the method also includes the step of reducing a data rate of the data carried over the at least one of the K channel wavelengths at the transmitting end in response to the determining step.
Further in accordance with a preferred embodiment of the present invention there is also provided a method of transmitting data carried over K out of N channels to be employed in an optical data communication system adapted to use N optical transmitters for transmitting information from a transmitting end of the communication system to a receiving end of the communication system using WDM, the method including detecting the K channels carrying data, routing the K channels to K optical transmitters for transmission of the data carried over the K channels by the K optical transmitters over K channel wavelengths corresponding to the K channels, wherein the K optical transmitters are selected from the N optical transmitters, determining that a quality of reception of data received over at least one of the K channel wavelengths at the receiving end is below a threshold, and reducing a data rate of the data carried over the at least one of the K channel wavelengths at the transmitting end in response to the determining step.
There is also provided in accordance with a preferred embodiment of the present invention a transmission method for use in a WDM optical data communication system, the method including generating data signals at different wavelengths in a wavelength range between 1.52 micrometer and 1.62 micrometer, generating control signals at a wavelength around 1.31 micrometer for controlling transmission of the data signals, and multiplexing the data signals and the control signals for transmission in a wavelength division multiplexed form.
Additionally, the method also includes the steps of detecting K channel wavelengths carrying the data signals, and routing the K channel wavelengths to a sub-group of K optical transmitters, selected from a group of N optical transmitters operating at channel wavelengths spaced at least S1 nm from each other, to provide a distribution of the K optical transmitters in which a highest number of optical transmitters in the sub-group transmit at channel wavelengths spaced at least S2 nm from channel wavelengths used by at least one nearest neighbor in the sub-group, where S2 is greater than S1.
There is also provided in accordance with a preferred embodiment of the present invention a switching unit for use in an optical data communication system which communicates data provided over N channels via N different channel wavelengths spaced at least S1 nm from each other, the switching unit including N data sensors, each operatively associated with one of the N channels, and each operative to generate an indication in response to detection of data carried over a channel associated therewith, a router operative to selectively route signals, and a controller operatively associated with the N data sensors and the router and operative to receive from K data sensors K indications indicating that a first sub-group of K out of the N channels carry data, and to provide to the router at least identification signals determining a second sub-group of K channels, the second sub-group of K channels corresponding to a sub-group of K out of the N channel wavelengths used by K optical transmitters to transmit the data carried over the K channels.
Preferably, the second sub-group of K channels is selected based on a selection of the corresponding sub-group of K channel wavelengths which provides a distribution of channel wavelengths characterized by that a highest number of channel wavelengths in the corresponding sub-group of K channel wavelengths are spaced at least S2 nm from at least one nearest neighbor channel wavelength in the sub-group of K channel wavelengths, where S2 is greater than S1. Further preferably, S2=S1*(N/K+1).
Preferably, the N data sensors, the router, and the controller are included in a single integrated circuit.
Additionally, the switching unit also includes a receiver operatively associated with the controller and operative to receive a reception indicator indicating that a quality of reception of data received over at least one of the K channel wavelengths at a receiving end is below a threshold, wherein the controller is also operative to determine a channel hop from the at least one of the K channel wavelengths to at least one unoccupied channel wavelength.
Additionally or alternatively, the controller is also operative to cause a reduction of a data rate of the data carried over the at least one of the K channel wavelengths in response to reception of the reception indicator.
Preferably, each of the N channel wavelengths is included in a wavelength range between 1.52 micrometer and 1.62 micrometer, and the receiver is operative to receive the reception indicator over a control channel wavelength operating at a wavelength around 1.31 micrometer. Further preferably, the controller is operative to determine the channel hop from the at least one of the K channel wavelengths to at least one channel wavelength operating at a wavelength around 1.31 micrometer.
Preferably, the router is operatively associated with N optical transmitters transmitting over the N channel wavelengths, and the router is also operative to selectively route signals for modulating at least one of input currents to and output beams of K out of the N optical transmitters under control of the controller, the K optical transmitters being associated with the K channel wavelengths.
Alternatively or additionally, the N channels are operatively associated with N optical receivers, the router is operatively associated with the N optical receivers and the router is also operative to selectively route signals received from at least some of the N optical receivers, and the controller is also operative to provide to the router identification signals determining K out of the N optical receivers associated with the K channel wavelengths.
Preferably, the controller is also operative to provide to the router the data carried over the K channels.
There is also provided in accordance with a preferred embodiment of the present invention transmission apparatus in a WDM optical data communication system, the apparatus including a plurality of optical transmitters generating optical signals in a wavelength range between 1.52 micrometer and 1.62 micrometer, each being operative to generate data signals at a different wavelength in the wavelength range, a control data optical transmitter generating optical signals at a wavelength around 1.31 micrometer and operative to generate control signals for controlling transmission of the data signals, and a wavelength division multiplexer operatively associated with the plurality of optical transmitters and the control data optical transmitter and operative to multiplex the data signals and the control signals for transmission in a wavelength division multiplexed form.
Additionally, the transmission apparatus also includes a switching unit operative, when some of the plurality of optical transmitters are not used to transmit the data signals, to selectively distribute the data signals between at least one of the control data optical transmitter and optical transmitters which are used to transmit the data signals.